A wide variety of implantable medical devices (IMDs) have been developed in order to monitor patient conditions and deliver therapy to the patient. An implantable pacemaker or cardioverter/defibrillator, for example, typically includes a hermetically sealed housing coupled to one or more leads that are surgically implanted inside a patient for short or long term therapy. The IMD may provide therapeutic stimulation to the patient or may deliver drugs or agents to the patient. Alternatively or additionally, the IMD can have sensing or monitoring capabilities. For example, the IMD may sense information within a patient and store the sensed information for subsequent analysis. Telemetry can be used to communicate sensed information from the IMD to an external medical device so that analysis can be performed. Also, in some cases, the sensed information may be used directly by the IMD to adjust or control the therapy that is delivered to the patient.
IMDs, such as pacemakers, often include an accelerometer for activity level sensing to support rate-responsive pacing. Conventionally, accelerometers are implemented to monitor patient movement and activity. For example, if the accelerometer indicates that the patient is moving, the pacemaker may increase the patient's heart rate to keep up with physiological demands of the body. Similarly, if the accelerometer identifies prolonged non-movement of the patient, the pacemaker can decrease the patient's heart rate.